This invention relates to rf switching circuits and, more particularly, to low spurious output, high speed rf switching apparatus.
Frequency synthesizers have a capability of generating signals at a number of selectable frequencies and are utilized in a variety of applications. According to one technique known as direct frequency synthesis, a number of individual oscillators are coupled through a switching circuit, or multiplexer, to the inputs of a mixer. The mixer generates sum and difference frequencies of its two inputs. By appropriate switching circuit inputs, the desired oscillators can be connected to the inputs of the mixer. Using this technique, n oscillators can generate n.sup.2 different output frequencies.
In many applications, such as in radar and electronic countermeasures, frequency synthesizers are required to generate a range of rf frequencies and must be switched between frequencies very rapidly. In addition, the synthesizer output must be relatively free of spurious, or unwanted, output signals. In practice, it has been difficult to satisfactorily meet all of the above requirements.
As is well known, rf signals are subject to stray coupling between adjacent circuit elements and to inadvertent coupling through conductors due to parasitic capacitances of circuit elements and inductance of ground conductors. Enclosure of rf circuitry in conductive housings is effective to contain radiated energy. However, connections to the conductive housing such as power and control connections provide a path for conduction of rf signals out of the housing. Such leakage signals usually appear at the output as spurious signals.
In the case of dc power connections, filtering of the rf signals is effective. In the case of high speed control signals, however, the frequency of the control signal can be close to the rf signal frequency. Construction of a filter to pass the control signal while blocking the rf signal is difficult. The filter response must be carefully tailored and typically requires a large number of poles. Such filters are large and expensive.
It is an object of the present invention to provide novel rf switching apparatus.
It is another object of the present invention to provide rf switching apparatus having low spurious output.
It is still another object of the present invention to provide frequency synthesizers having low spurious output and a capability of high speed switching between frequencies.
It is yet another object of the present invention to provide rf switching apparatus wherein the switching control signal is optically coupled to the rf switching circuit.